Technique for monitoring SO3, H2 SO4 in exhaust gases containing SO2

ABSTRACT

The SO 3 , H 2  SO 4  content of a gas environment containing SO 2  and H 2  O is measured by cooling the gas to a temperature to convert SO 3  in the presence of H 2  O to H 2  SO 4  to effectively separate SO 3 , H 2  SO 4  from SO 2  to permit the individual measurements of SO x  (SO 2  +SO 3 ), SO 2  and SO 3 , H 2  SO 4 .

BACKGROUND OF THE INVENTION

Solid electrolyte detectors for the on-line monitoring of sulfur-bearing pollutants SO₂, SO₃, i.e. SO_(x), have recently been developed and are described in detail in the issued Canadian Pat. No. 1,040,264, entitled "Solid State Sensor For Anhydrides", issued Oct. 10, 1978, which is assigned to the assignee of the present invention and incorporated herein by reference. This inventive concept is the subject matter of pending U.S. Patent application Ser. No. 718,511 now U.S. Pat. No. 4,282,078. The operation of the SO_(x) detector described in the above-referenced patent and patent application is based on potentiometric measurements across a solid electrolyte element of potassium sulfate (K₂ SO₄), wherein accurate measurements of sulfur-bearing pollutants over a concentration range of 0.1 parts per million to 10,000 parts per million can be realized. The sensor thus described is uniquely sensitive to SO_(x). The presence of other common pollutants such as CO₂, CH₄ and NO_(x) does not interfere with the SO_(x) measurements.

SUMMARY OF THE INVENTION

The disclosed technique utilizes the condensation property of SO₃ as H₂ SO₄ (liquid) and uses this to quantitatively separate SO₃ and H₂ SO₄ from SO₂ by appropriate temperature control. In accordance with the disclosed technique, the above-referenced solid electrolyte electrochemical cell detector can be made to respond to total SO_(x), or separately to SO₂ or SO₃, H₂ SO₄.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more readily apparent from the following exemplary description in connection with the accompanying drawings:

FIG. 1 is a graphical illustration of the conversion of SO₂ to SO₃ ;

FIG. 2 is a graphical illustration of the conversion of SO₃ to H₂ SO₄ ;

FIG. 3 is a graphical illustration of acid dew point as a function of H₂ SO₄ concentration;

FIG. 4A is a schematic illustration of an embodiment of the invention;

FIG. 4B is a graphical illustration of the operation of the embodiment of FIG. 4A;

FIG. 5A is a schematic illustration of an alternate embodiment of the invention;

FIG. 5B is a graphical illustration of the operating of the embodiment of FIG. 5A;

FIG. 6A is a second alternate embodiment of the invention;

FIG. 6B is a graphical illustration of the embodiment of FIG. 6A; and

FIG. 7 is a graphical illustration of the moisture content of a gas as a function of the acid dew point and the H₂ SO₄ concentration.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The SO_(x) solid electrolyte electrochemical cell sensor described in the above-referenced patent and patent application, and employed herein, consists of a solid electrolyte member having electrodes disposed on opposite surfaces thereof wherein the electrodes are typically platinum. The cell can be represented as follows:

    SO.sub.3 (P.sub.1), O.sub.2 (P.sub.1 ') Pt/K.sub.2 SO.sub.4 /Pt, O.sub.2 (P.sub.2 '), SO.sub.3 (P.sub.2)

wherein the electrode reactions correspond to:

    SO.sub.3 +1/2O.sub.2 +2e.sup.- ⃡SO.sub.4.sup.═

The EMF of this solid electrolyte electrochemical concentration cell is defined by the Nerst equation: ##EQU1## where:

E=EMF;

R=gas constant;

T=temperature (°K.); and

F=Faraday.

Due to the following equilibrium relationship

    SO.sub.3 ⃡SO.sub.2 +1/2O.sub.2

the cell will respond to either SO₂ or SO₃ and the species involved in the cell reaction will depend on the temperature of the cell. The temperature dependence of the SO₂ -SO₃ equilibrium is shown in FIG. 1. The equilibrium in the vicinity of the platinum electrode, considering platinum to be catalytic, is reached very rapidly.

The operation of the cell in accordance with the Nernst equation requires the establishment of a stable SO_(x) reference at a reference electrode of the cell such that at a given cell temperature the change in EMF produced by the cell is a function of the SO_(x) of the monitored gas mixture at the opposite, or sensing, electrode. A change in the EMF is indicative of a change in the partial pressure of SO_(x) at the sensing electrode. The Nernst equation for monitoring the partial pressure of SO₂ in a monitored gas mixture present at the sensing electrode of the cell is represented as follows: ##EQU2## The operating temperature for the cell is typically between 600° and 1000° C.

In accordance with the above mode of operation of the previously disclosed SO_(x) responsive solid electrolyte cell it is not possible to discriminate between SO₃ and SO₂ and in fact the measurement provided is an indication of the total SO_(x) concentration. Furthermore, as is evident from the above representation of the Nernst equation, the oxygen content of the monitored gas must be known or the oxygen pressures pO₂ and pO₂ ' maintained equal.

It has been determined that SO₃ reacts spontaneously and reversibly with water vapor to form H₂ SO₄. The thermodynamic equilibrium for this gas phase reaction as a function of temperature is shown in FIG. 2.

Although the equilibrium is a function of the water vapor content of the gas, it has been determined that for typical ambient or stack humidities, the equilibrium will be entirely in the direction of H₂ SO₄ at temperatures below 200° C. Below this temperature H₂ SO₄ will condense. The condensation temperature, i.e. acid dew point, is dependent on the concentrations of H₂ SO₄ and H₂ O in the gas mixture. The relationship between the dew point temperature and H₂ SO₄ concentration for several water vapor concentrations is shown in FIG. 3. It is seen that if the gas mixture is cooled to approximately 100° F., virtually all of the H₂ SO₄ will be condensed even at water vapor concentrations typical of the ambient air (approximately 1.5 vol. %). SO₂ will not condense at this temperature and thus an effective means of separating SO₂ from SO₃ and H₂ SO₄ is provided. This principle forms the basis for the use of the Goksoyr-Ross coil method for determining the SO₃, H₂ SO₄ concentration in stack gases. The information illustrated in FIG. 4 can be verified in an article appearing in Chemical Engineering Progress, Vol. 71 (1974) by Verhoff and Banchero. The above-referenced Goksoyr-Ross coil method is the subject of an article appearing in the Journal of Fuel Instrumentation, Vol. 177 (1962).

It should be noted however, that the monitored gas mixture cannot be cooled below the water dew point inasmuch as condensation of water in the condenser will result in conversion of SO₂ to SO₃. The novel technique disclosed herein involves the combination of the above-disclosed technique for the controlled cooling of the sample of monitored gas and the use of a solid electrolyte SO_(x) detector to produce a device for real time monitoring of the presence of SO₃, H₂ SO₄ in a monitored stack gas. In a stack gas SO₃ and H₂ SO₄ are closely associated and distinct from SO₂. It will be apparent from the following discussion that numerous variations of the disclosed technique can be employed to adapt the device for use with gases of widely varying SO₃ content.

The embodiments of FIGS. 4A, 5A and 6A are those of sampling systems wherein a sample of the monitoring gas, i.e. ambient air, exhaust gas, etc., is drawn into a measuring apparatus.

The embodiment of FIG. 4A illustrates an SO₃, H₂ SO₄ monitoring system suitable for use in monitoring stack gases with relatively high SO₃ content, i.e. greater than 10 parts per million. The detector system 10 of FIG. 5A includes a heated filter 20, a hot/cold leg gas bypass apparatus 30 and an SO_(x) detector 40. A sample of the monitored gas environment G is initially passed through a prefilter 20 heated to a temperature of about 300° C. to remove particulates from the gas sample before the gas sample reaches the detector 40. By switching the sample of monitored gas G between the hot leg 32 at a temperature of about 300° C., and the cold leg 34 at a temperature of about 40° C., of the gas bypass apparatus 30, total SO_(x) and SO₂ can be separately and alternately measured by the detector 40. The difference between the measured concentration of SO_(x) and SO₂ is a measurement of the SO₃, H₂ SO₄ concentration in the monitored gas G. The SO₃ content is quantitatively condensed as H₂ SO₄ in the cold leg 34. The operation of the system 10 on the basis of 5 minute cycle times between the sample gas flow through the hot leg 32 and the cold leg 34 is graphically illustrated in FIG. 4B. The EMF output signals sequentially generated by the detector 40 in response to the alternate supply of the monitored gas through the hot and cold legs 32 and 34 is transmitted to a conventional subtracting circuit 50. The circuit 50 subtracts the two signals and generates an output indicative of the SO₃, H₂ SO₄ concentration of the monitored gas G.

The detector 40 consists of a solid electrolyte electrochemical cell 42 comprising a solid electrolyte member 43 having a sensing electrode 44 disposed on one surface and a reference electrode 45 disposed on the opposite surface. The solid electroltye cell 42, which is constructed as described above in accordance with the teachings of the above-referenced patent, is positioned within a housing 48 so as to isolate the monitored sample gas G which contacts the sensing electrode 44 from the reference electrode 45. An SO_(x) stable reference is provided in contact with the reference electrode such that the operation of the cell 42 in accordance with the above Nernst equation. The temperature of the detector 40 is maintained constant by the heater 47. The monitored sample gas introduced to the sensing electrode 44 via the inlet tube 35 is removed from the housing via a pump system 60.

A technique for monitoring H₂ SO₄ in atmospheric gas where little, if any, SO₃ is present is illustrated in FIGS. 5A and 5B. The gas G is initially cooled by a cooler 73 to separate H₂ SO₄ from SO₂. The H₂ SO₄ is collected by a filter on trap 70. The remaining gas volume is removed by the pump 60 through a gas meter 61 which measures the volume of gas sampled. This information, in accordance with FIG. 5B, is used in determining the SO₃ concentration of the gas sampled during a predetermined period of time. Following a collection period of several hours or days, the filter 70 is heated through the operation of a temperature controller 71 and an over member 72 to volatilize the trapped H₂ SO₄ and produce H₂ SO₄. The H₂ SO₄ is then transmitted to the SO_(x) detector 80 consistingof the solid electrolyte electrochemical cell 82 positioned within housing 84 and an EMF signal is generated by the cell 82 which is indicative of the H₂ SO₄ concentration collected over a period of time. Referring to the graphical illustration of FIG. 5B, the area under the H₂ SO₄ response curve is indicative of the amount of acid collected from a known volume of ambient gas. As indicated with respect to the embodiment of FIG. 5A, a heated prefilter 75 heated to a temperature of about 150° C. or higher can be included upstream of the filter 70 to retain solid particulate matter.

Yet another embodiment of the SO₃, H₂ SO₄ detection technique is illustrated in FIGS. 6A and 6B. In this implementation the inlet tube 90 to the SO_(x) detector 98 includes a heated prefilter 89 and a variable temperature trap 91 consisting of an oven 92 and a temperature control 93. The prefilter 89 is heated to a temperature of about 150° C. or higher. At temperatures of approximately 150° F., both SO₂ and SO₃, H₂ SO₄ through the trap 91 and are detected by the sensor 98 to provide a total SO_(x) indication. As the temperature of the trap 91 is lowered via the temperature controller 93, a temperature will be reached where SO₃ begins to be converted to H₂ SO₄ and condensing begins in the trap 91. This temperature is the H₂ SO₄ acid dew point. If cooling is continued, a temperature is reached where all of the SO₃ is converted to H₂ SO₄ and is condensed out of the sample gas stream G entering inlet tube 90. The total change in the EMF signal generated by the detector 98 is equivalent to the SO₃, concentration of the sample gas G. This information regarding the acid dew point temperature and SO₃, H₂ SO₄ concentration of the monitored gas G can be used to determine the moisture content of the monitored gas G in accordance with the graphical illustration of FIG. 7.

It is apparent from the above that a single apparatus can be employed to measure four important parameters of an exhaust gas, i.e. SO₂ concentration, SO₃ concentration, H₂ O concentration and H₂ SO₄ acid dew point temperature.

In each of the monitoring systems described above, the choice of materials should eliminate those which would act as catalysts for the conversion of SO₂ to SO₃. While most metals promote oxidation of SO₂, quartz and porcelain can be considered as inert materials for use as monitored gas input lines, variable temperature coils and traps, etc. A variety of filter materials including quartz fiber, nucleopore, and Teflon have been found to be inert, efficient collectors for H₂ SO₄ in the presence of SO₂. 

We claim:
 1. A method of monitoring SO₃, H₂ SO₄ in the presence of SO₂ in a monitored gas environment containing H₂ O comprising the steps of:measuring the combined SO₂ and SO₃ content of a sample of the monitored gas environment; separating SO₂ from the SO₃, and H₂ SO₄ by cooling the sample to a temperature which is sufficient to convert SO₃ in the presence of H₂ O to H₂ SO₄, but which is not sufficient to condense SO₂ ; measuring the SO₂ content of the sample following said separating step; and determining the SO₃, H₂ SO₄ content of the monitored gas environment on the basis of the difference between the measurement of the combined SO₂ and SO₃, H₂ SO₄ and the measurement of the SO₂ following said separating step.
 2. A method as claimed in claim 1 wherein the temperature for converting SO₃ in the presence of H₂ O to the H₂ SO₄ by said cooling is above the water dew point.
 3. A method as claimed in claim 1 wherein the temperature of said cooling is less than approximately 200° C.
 4. A method as claimed in claim 1 further including the step of filtering said sample of the monitored gas prior to said cooling step to remove particulates from the gas sample.
 5. A method as claimed in claim 1 further including the step of sensing the temperature at which the cooling begins to convert the SO₃ to H₂ SO₄, said temperature corresponding to the H₂ SO₄ acid dew point.
 6. A method as claimed in claim 1 further including the steps of determining the temperature at which the conversion of SO₃ to H₂ SO₄ begins, said temperature corresponding to the H₂ SO₄ acid dew point, and determining the moisture content of the monitored gas environment as a function of the acid dew point and the SO₃ content of the monitored gas environment.
 7. A method of monitoring SO₃, H₂ SO₄ in the presence of SO₂ in a monitored gas environment containing H₂ O comprising the steps of:sampling the monitored gas environment; separating SO₂ and SO₃, H₂ SO₄ by cooling the sample of the monitored gas environment to a temperature which is sufficient to convert SO₃ in the presence of H₂ O to H₂ SO₄, but which is not sufficient to condense SO₂ ; trapping the resulting H₂ SO₄ over a period of time; measuring the volume of monitored gas environment which is sampled during said period of time; heating the H₂ SO₄ collected by said trapping step to produce H₂ SO₄ vapor; and measuring the H₂ SO₄ vapor produced by said heating step in terms of said volume of monitored gas environment to determine the SO₃, H₂ SO₄ content of saidmonitored gas environment. 